1. Field of the Invention
The present invention relates to a reversible thermosensitive recording material, more particularly to a reversible thermosensitive recording material comprising a reversible thermosensitive recording layer of which transparency or color tone is reversibly changeable depending upon the temperature thereof, thereby recording information therein and erasing the information therefrom repeatedly as desired.
2. Discussion of Background
Recently attention has been paid to a reversible thermosensitive recording material capable of temporarily recording images therein and erasing the same therefrom when such images become unnecessary. For example, as disclosed in Japanese Laid-Open Patent Applications 54-119377 and 55-154198, there are conventionally known reversible thermosensitive recording materials in which an organic low-molecular-weight material such as a higher fatty acid is dispersed in a matrix resin such as a vinyl chloride-vinyl acetate copolymer with a low glass transition temperature (Tg) of 50 or 60.degree. C. to less than 80.degree. C.
Such conventional reversible thermosensitive recording materials, however, have the shortcomings that the recording layer is distorted while images are formed and erased repeatedly using a heating element such as a thermal head, so that image density and image contrast are significantly decreased while in use.
In order to solve the above-mentioned first problem and to improve the durability of the reversible thermosensitive recording material during the repeated operations of image formation and image erasure, the inventors of the present invention have already proposed a reversible thermosensitive recording material as disclosed in Japanese Laid-Open Patent Application 5-38872. This recording material comprises an epoxy resin as a matrix resin for use in the reversible thermosensitive recording layer. This method is capable of solving the above-mentioned first problem to some extent, but it is still insufficient for practical use.
There is proposed a reversible thermosensitive recording material in Japanese Laid-Open Patent Application 5-085045. According to this application, since a thermosetting resin comprising a hydroxy-modified vinyl chloride-vinyl acetate copolymer and an isocyanate compound is used as the matrix resin in the reversible thermosensitive recording layer, the heat resistance and mechanical strength of the reversible thermosensitive recording material are improved. As a result, the durability of the recording material can be improved while the image formation and erasure is repeatedly carried out using the thermal head.
Generally, when there is employed a reversible thermosensitive recording material comprising a matrix resin and an organic low-molecular-weight material dispersed in the matrix resin, the reversible thermosensitive recording layer assumes a transparent state within a specific temperature range (hereinafter referred to as a transparency temperature range), and such a transparent state is changed to a white opaque state at a temperature higher than the above-mentioned transparency temperature range. The mechanism of image formation and erasure in the recording layer is based on the above-mentioned change of the states. To reversibly switch the states between the transparent state and the white opaque state by the application of heat to the reversible thermosensitive recording material, it is especially necessary that the above-mentioned transparency temperature range be wide to some extent and be stable for an extended period of time.
However, with respect to the conventional reversible thermosensitive recording material as disclosed in Japanese Laid-Open Patent Application 5-085045, the transparency temperature range becomes narrow with time. The reason for this is that the curing degree of the thermosetting resin for use in the reversible thermosensitive recording layer changes with time. To be more specific, the curing degree of the thermosetting resin obtained at the formation of the reversible thermosensitive recording layer changes with time. As a result, there occurs the second problem that it is impossible to erase the image at the same image erasure temperature, as initially determined, with the lapse of time. Therefore, the determination of the image erasure temperature becomes complicated after repeated operations.
Furthermore, in the case where a resin, in particular, a resin comprising as the main component a vinyl chloride resin, is subjected to cross-linking for the formation of the reversible thermosensitive recording layer, the corrosion of a metal-deposited light reflection layer (hereinafter also referred to as a light reflection layer), which is interposed between the support and the reversible thermosensitive recording layer, cannot be avoided. Accordingly, the image contrast is lowered and the recording layer changes to red.
In order to eliminate the above-mentioned first and second problems, the inventors of the present invention have already proposed a reversible thermosensitive recording material, as disclosed in Japanese Laid-Open Patent Application 7-172072. The reversible thermosensitive recording layer of the above-mentioned recording material shows a thermal pressure level difference of 40% or less and a thermal pressure level difference change ratio of 70% or less. In this case, not only the heat resistance and the mechanical strength of the thermosensitive recording layer can be upgraded and the durability of the recording material can be improved when repeatedly used together with a thermal head, but also the stable transparency temperature range can be obtained for an extended long period of time. By using the above-mentioned reversible thermosensitive recording material, the previously mentioned conventional problems can be solved to a certain extent.
This kind of reversible thermosensitive recording material can be repeatedly used. It means that the recording material is operated or allowed to stand under a variety of circumstances. When the reversible thermosensitive recording material is allowed to stand for a long period of time under the circumstances of high humidity, for example, at 40.degree. C. and 90%RH, or at 35.degree. C. and 85%RH, the reflection densities of a white opaque image portion and a transparent background portion change with time. In particular, the reflection density of the white opaque image portion gradually increases. As a result, the image contrast is lowered, and visual recognition of the image becomes difficult. This problem is a new subject with respect to the reversible thermosensitive recording material, and the countermeasure against this problem has not yet been discovered.
Furthermore, this kind of reversible thermosensitive recording material has another new problem. Namely, an image portion and a background portion tend to change to red when the recording material is allowed to stand at high temperature, for example, at 50.degree. C. to 70.degree. C., for a long period of time or while the image formation and erasure is repeated many times using a thermal head. This problem has not yet been solved.